The present invention relates generally to an anti-skid brake control system for an automotive brake system which optimizes vehicular braking characteristics and minimizes braking distance. More specifically, the invention relates to measurement of vehicle speed deceleration for use in deriving a target wheel speed relative to the vehicle speed.
As is well known, optimum braking characteristics are obtained when braking pressure or force can be so adjusted that the peripheral speed of the wheels during braking is held to a given ratio, e.g. about 80% to 85%, of the vehicle speed. This practice is believed to be particularly effective when road conditions and other factors are taken into consideration. On the other hand, if the wheel speed/vehicle speed ratio is maintained higher than the above-mentioned optimal ratio, e.g., 80% to 85%, braking distance may be prolong due to a lack of braking pressure. On the other hand, if the braking pressure is so adjusted as to maintain the wheel speed/vehicle speed ratio with respect the vehicle speed less than the aforementioned optimal ratio, the vehicle wheels may lock and skid, resulting in an unnecessarily long braking distance due to reduced traction. In practice, it is very difficult to precisely adjust the braking pressure so that the wheel speed is held to the given optimal ratio to the vehicle speed.
In the practical anti-skid brake control operation, braking pressure in one or more wheel cylinders is adjusted by cyclically increasing and decreasing the braking pressure in the wheel cylinder. The anti-skid control system generally decreases braking pressure when the wheel deceleration value becomes less than a given deceleration threshold, which is so chosen as to prevent the wheel from skidding, and increases braking pressure when the wheel acceleration value is greater than a given acceleration threshold. In this conventional anti-skid brake control procedure, wheel speed does not remain at an optimal relationship to the vehicle speed for a satisfactorily long period of time.
U.S. Pat. No. 3,637,264, issued on Jan. 25, 1972 to Leiber et al discloses an Antilocking Control for Pressure Actuated Brakes. The pressure of the brake-actuating fluid in an antilocking brake control system is varied by pulsing the control valve or valves for durations which are varied to be greater or lesser than the period of that limiting frequency above which the brake system cannot respond. In the former case, a rapid increase in fluid pressure or a rapid decrease in fluid pressure occurs, whereas in the latter case, a less rapid average or net increase or decrease occurs in the fluid pressure to which the brake system responds. These conditions are controlled in dependence on the rotational behavior of the vehicle wheel or wheels and more especially in dependence or predetermined changes in angular velocity of the wheel. Moreover, either variation in pulse duration at a fixed frequency or variation in frequency at a fixed pulse duration may be effected during high-frequency pulsing so as further to alter the net increase or decrease in fluid pressure. This further alternation is effected as a function of time from the beginning of the high-frequency pulsing.
In addition, Published Japanese Patent Application (Tokkai) Showa No. 51-89096, published on Aug. 4, 1976 discloses a system similar to the above. The fluid pressure in the wheel cylinder is increased in a stepwise manner. Duration of the increase of the fluid pressure is adjusted in accordance with the rate of increase of the fluid pressure in one or more preceding steps.
Another approach for deriving the target wheel speed has been disclosed in U.S. Pat. No. 4,430,714, issued on Feb. 7, 1984, to the common inventor and commonly assigned. In the shown system, wheel speed is sampled and held every time wheel acceleration drops below a predetermined deceleration threshold. The intervals between samplings of the wheel speed value are measured. On the basis of the difference between the wheel speed in the current skid cycle and that measured in the immediately preceding skid cycle and the measured interval between samplings of the wheel speed in the current and immediately preceding cycles, wheel speed deceleration rate is derived. The derived wheel deceleration rate is used in deriving the target wheel speed for the next skid cycle of brake control operation.
This conventional technique cannot derive the target wheel speed with sufficient precision. In particular, variations in road surface friction during anti-skid brake control make it difficult to hold the brake pressure near the lock pressure at which the vehicle wheels start to skid. For instance, if road surface friction decreases, thus decreasing the lock pressure, the wheels may be induced to skid if the target wheel speed derived by way of the above technique is adhered to. On the other hand, if the road friction increases, the wheel speed deceleration rate derived by way of the above procedure may become unnecessarily great, resulting in significant fluctuations in skid control.